Target electrode assembly for an electron discharge device



Nov. 22, 1966 R. B. RANDELS 3,287,585

TARGET ELECTRODE ASSEMBLY FOR AN ELECTRON DISCHARGE DEVICE Filed March15, 1965 Fig.5. k38

INVENTOR Robert B. Rondels BY F|g.4.

ATTORNEY United States Patent 3,287,585 TARGET ELECTRODE ASSEMBLY FOR ANELECTRON DISCHARGE DEVICE Robert B. Randels, Painted Post, N.Y.,assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., acorporation of Pennsylvania Filed Mar. 15, 1963, Ser. No. 265,425 8Claims. (Cl. 313-89) This invention relates to an improved targetelectrode assembly and more particularly to an improved assembly inwhich an electric charge pattern or image is stored on a very thindielectric member or membrane.

One particular application of this invention is within an image orthicontype pickup tube. The image orthicon tube consists of a photocathodeonto which light from an input scene is directed and which emitselectrons in response to the light. The photoelectrons are in turnaccelerated and focused onto a thin dielectric target of a material suchas glass. The energy of the electrons directed onto the dielectricstorage target is sufficient to cause secondary emission from thedielectric target and thereby establish a potential pattern on theopposite side of the dielectric target representative of the visibleimage directed onto the photocathode. This pattern established on theopposite side of the dielectric target with respect to the photocathodeis read out by an electron beam as is well known in the art to derive avideo signal representative of the visible information directed onto thepickup tube. It is the customary practice to provide a fine mesh screenoverlying or adjacent the dielectric target surface facing thephotocathode for collecting the secondary electrons emitted from thedielectric target in response to bombardment by the photoelectrons.

In tubes of this type, the fine mesh screen collector electrode and thedielectric target are normally mounted together in a unitary target meshassembly. The amount of spacing between the dielectric target and thecollect-or mesh may vary in different tubes in order to obtain certaincharacteristics for particular applications. It is however desired thatthere be a uniform spacing between the dielectric target and thecollector mesh over the entire surface. The normal spacing within theimage orthicon is about 1 to 3 mils.

The dielectric target is normally of glass and is about 0.2 to 0.4 milin thickness. Due to the manner in which the target is formed, it hasbeen difficult in the past to provide a satisfactory target assembly inwhich the collector mesh is positioned at'a uniform distance withrespect to the dielectric target. The result of the nonuniform spacingis, of course, a variation in video signal intensity from one point toanother as the glass target is scanned by the electron beam. The priorart practice of mounting the collector mesh and the dielectric membraneabout their periphery resulted in similarities to a drum head mount. Thetube, when subjected to mechanical vibration, exhibited rapid variationsin capacity between the mesh and the membrane. This produced an unwantedmodulation of the electrical signal output, not related to the chargepattern established on the membrane. This undesirable effect is referredto as microphonics. This problem is recognized within the industry andseveral solutions have been arrived at in an attempt to solve thismicrophonics problem. Most of these solutions have several disadvantagesin the form of complexity, expense and creating other electricalproblems.

It is accordingly an object of this invention to provide an improvedelectrode assembly.

It is another object of this invention to provide an improved target andmesh assembly which provides uniform operation over the entire targetsurface.

It is another object of my invention to provide an improved target andscreen assembly which is substantially free from microphonics.

It is another object of my invention to provide an improved target andscreen assembly to prevent mircophonics due to physical forces appliedto the tube.

In accordance with my invention, I provide a target assembly in which athin spacer member having a substantially rectangular opening isprovided between the collector mesh and the dielectric membrane. Inaddition, the spacer member provided between the collector mesh and thedielectric member is of such a construction as to provide a plurality ofsupport surfaces for the membrane.

Further objects and advantages of the invention will become apparent asthe following description proceeds. The features of novelty whichcharactirize the invention will be pointed out in particularity inclaims annexed to and forming a part of the description.

For a better understanding of the invention, reference may be had to theaccompanying drawings, in which:

FIGURE 1 is a cross-sectional view of an image orthicon pickup tnbeembodying the teachings of my invention;

FIG. 2 is an exploded view of the target assembly structure embodied inFIG. 1;

FIG. '3 is a sectional view of the target assembly structure embodied inFIG. 1;

FIG. 4 is a face or front view of the target assembly embodied in FIG.1;

FIG. 5 is an enlarged and sectional view of a portion of the targetassembly taken along line V-V of FIG. 3;

Referring now to FIG. 1, there is illustrated an image orthicon pickuptube comprising an envelope 10 of a suitable material such as glass,having an enlarged or image section 12 at one end thereof. Within theother tubular portion 14 of the envelope 10, there is mounted aconventional image orthicon electron gun structure 16 for providing anelectron beam of low velocity which is focused and accelerated by ananode electrode formed as a wall coating 18 on the inner surface of theenvelope 10. A decelerating electrode 21 reduces the velocity of theelectron beam so that it approaches a target assembly 39 including athin glass target member 30 at practically zero velocity. Anelectromagnetic coil (not .shown) surrounds the tubular envelope 10 andprovides a focusing field aligning the electron beam perpendicularly tothe surface of the glass target 30. A neck yoke (not shown) is alsoprovided to provide a pair of scanning or deflecting fieldsperpendicular to each other and to the axis of the tube for causing theelectron beam from the electron gun 16 to scan the surface of the glasstarget 30. The pairs of coils of the neck yoke are connected inappropriate circuits (not shown) which are provided with pulses as iswell known in the art for furnishing the scanning magnetic fields.

The enlarged portion 12 of the envelope 10 is closed by a lighttransmissive face plate portion 20. A photocathode surface 22 isprovided on the inner surface of the face plate 20. A scene, which is tobe televised, is optically focused upon the photocathode surface 22 andcauses the emission of photoelectrons from the photocathode 22 inproportion to the amount of light striking the elemental areas of thephotocathode 22. The photoelectron emission from the photocathodesurface 22 is focused on the target 30 by a magnetic field provided by acoil (not shown) around envelope 10 and the accelerating fields ofelectrodes 24 and 26. The photoelectrons are accelerated to such anextent that they strike the surface of a glass target 30 with suificientenergy to be above the first crossover potential of the target andcreate a greater number of secondary electrons than incident primaryelectrons. A fine collector mesh screen 40 is closely spaced from thesurface of the glass target 30 and is maintained during the tubeoperation at a potential up to several volts positive relative to thecathode potential of the electron gun 16.

The low velocity electron beam of the electron gun 16 scanning thesurface of the target 39 will bring the potential of the target toapproximately that of the cathode of the gun. At this potential, thescanning electron beam will not land on the surface of the target 30 butwill be electrostatically reflected back along the axis of the tube 10.The secondary emission caused by the bombardment of the target 30 by thephotoelectrons from the photocathode 22 will cause the surface of theglass target 30 facing the photocathode 22 to charge in a positivedirection so as to provide a positive charge pattern corresponding tothe light image directed onto the photocathode 22. Due to the extremethinness of the glass target 30, positive charges on the image side ofthe target will set up a corresponding positive potential pattern on thereading or the electron gun side of the target. As the electron beamfrom the gun 16 scans the surface of the target 30, electrons from thebeam will be drawn to the positive areas of the target surface and willbe deposited and will neutralize the positive potential pattern on theglass target 30. This will tend to charge the surface of the target 30to cathode potential. The remainder of the beam unnecessary to returnthe surface of the target 30 to cathode potential will be reflected backtoward the electron gun 16. The negative charges deposited on the gunside of the target 30 will unite by conduction through the glass target30 with the positive charges on the photocathode side of the target 30.The electron beam from the gun 16 will be modulated by the removal ofelectrons deposited upon the surface of the glass target and inaccordance with the positive charge pattern formed thereon. Themodulated return beam is returned and will strike a first dynode 28 of amultiplier section surrounding the electron gun 16. This modulatedreturn is then amplified by the multiplier sections and collected on acollector electrode which is in turn connected to any appropriatecircuit to form the video signal. The above description is substantiallyconventional image orthicon operation and the applicant has describedhis invention incorporated in such a device in order to adequatelyexplain and set forth the invention.

Referring now to FIGS. 2, 3, 4 and for a more detailed description ofthe target assembly 39. The target assembly 39 is comprised of thecollector electrode 40 and the glass membrane target 30. The collect-or40 is comprised of a fine wire mesh 42 stretched across a circularaperture 44 provided in the annular mesh support ring 46.

The mesh 42 may be of a suitable material such as copper having athickness of about 0.5 mil and having about 600,000 apertures per squareinch. The support ring 46 may be of a suitable material such as an alloyof steel, iron and aluminum. The ring 46 includes a surface 43 facingthe photocathode 22 which is substantially planar. The other surface 45of the ring 46 includes an inner circular ridge or projection 47. Anannular depressed surface 49 is positioned adjacent the rib 47 and onthe same surface 45 of the ring 46 and extends completely around thering 46. The mesh screen 42 is mounted on the support ring 46 by placingthe peripheral edge of the mesh 42 over the depressed surface portion 49and welding a weld ring 50 over the mesh 42 to the depressed surfaceportion 49. The weld ring 50 may be of stainless steel and is spotwelded to the support ring 46. The remaining surface portion 51 of thesurface 45 lies in about the same plane as the exposed surface of theweld ring 50. The rib 47 extends above this plane and the mesh 42 isstretched over the rib 47. The glass target 30 is also mounted or fixedat its edge on a support ring 32 with the.

ring 32 having an inturned flange 34 having a circular aperture 36 towhich the glass membrane or storage film 38 is sealed. The ring 32 is ofa suitable material such as nickel iron and the technique of fabricatingthe glass and scaling to the ring is not a part of this invention and iswell know in the art.

Positioned between the collector mesh assembly 40 and the glass target30 is a spacer member 60 of a suitable material such as aluminum. Thespaced member 60 is an annular disc-like member having a substantiallyrectangular aperture 64. The spaced 60 as indicated in FIGS. 3 and 4 issuch as to provide an outer annular portion 62 lying in a first plane,an inner annular .portion 66 having the rectangular opening 64 lying ina second plane parallel to said first plane with an interconnectingportion 68. The space between the first and second planes is about 5mils. The spacer 60 is positioned on the surface 45 of the ring 46 asshown in FIG. 3 with the outer annular portion 62 in contact with one orboth the surface portion 51 and the weld ring 50. The flange portion 34of the target support ring 32 is in contact with the other surface ofthe outer annular portion 62. The annular interconnecting portion 68lies between the inner edge of the flange portion 34 of ring 32 and therib 47 of the ring 46. The assembly when clamped together by suitableclamping means utilizes the inner annular portion 66 and especially itsedge 64 as the spacer means between the membrane 38 and the mesh 42.

The clamping means may consist of an outer clamp ring 70 provided withlugs '72, an annular resilient member 74 resting on the surface 43, aninner clamp ring 76 resting on the resilient member 74, and the lugs 72which may be bent over to form an assembly as shown in FIG. 3. Otherclamping means are known to those skilled in the art.

The spacer 60 may be formed using rubber die techniques from aluminumfoil of about 1.8 mils in thickness. By using a foil of this thickness,it is found that a spacing of about 2.2 mils is provided betweenmembrane 38 and the mesh 42. By forming the spacer 60' in this manner,

7 a support is provided as illustrated in FIG. 5. This in effectprovides a plurality of point supports for the membrane 38. In thespecific embodiment shown, the aperture edges 64 of the spacer 60 formsubstantially rectangular aperture with smooth edges. It is possible tohave an irregular edge to provide small tongues or projections extendinginwardly. It is also possible to deform the area adjacent the edges toprovide a plurality of support surfaces.

It is found that a spacer of a thickness of about 1.8 mils provides awavy or undulating support surface with a plurality of support surfaces.The support surfaces alternate between the mesh 42 and the membrane 38.The points of support of the membrane 38 are separated by at least 15mils.

The resulting structure provides a separation spacer between themembrane 38 and the collector mesh 42 such as to be substantiallyrectangular in shape and the dimensions of the rectangular opening 64are slightly greater than the electron image directed onto the target30* from the photocathode 22. The result of the rectangular shapedspacer is that vibrations of the tube when transmitted to the targetwill generate only high frequency modes which will be quickly dampedout. The target glass 30 is stretched over the rounded shoulder of thespacer and rib 47. The random slight variations of the spacer 60 asshown in FIG. 5 from the perfect plane will produce points of contact tothe target glass 38 and the mesh 42 and thereby form nodal points thusassuring that only very high harmonics of the fundamental vibrationfrequency can easily be excited. These high harmonics will be quicklydamped out. If lower frequencies are excited, then these contact pointswill induce mode conversion and thus cause the vibration to be rapidlydamped out.

While there has been shown and described what are at present consideredto be the preferred embodiments of the invention, modifications theretowill readily occur to those skilled in the art. It is not desiredtherefore that the invention be limited to the specific arrangementshown and described and it is intended to cover in the appended claimsall such modifications as fall within the true spirit and scope of theinvention.

I claim as my invention:

1. A target assembly for an electron discharge device comprising atarget member including a supporting ring and a dielectric membranesupported thereon, a mesh electrode member including a support ring anda mesh supported thereon, a spacer member positioned between said targetmember and said mesh electrode, said spacer member in the form of anannular member having an outer periphery similar to said target membersupport ring and said mesh support ring, said spacer member having aninner periphery defining a substantially rectangular aperture of smallerarea than the inner area of said target support ring, the innerperiphery of said spacer lying just outside the active area of saidtarget, said spacer member of a thickness of about 2 mils and havingalternate support surfaces with respect to said mesh and membrane.

2. A storage target assembly for an electron discharge device comprisinga target member including a support .frame and a storage film supportedthereon, an electrode member including a support frame and a gridsupported thereon, an annular spacer member positioned between saidtarget member andsaid electrode member, said spacer member including anouter portion positioned between said target support frame and saidelectrode member support frame and an inner portion forming asubstantially rectangular opening, the edges of said opening havingalternate spacing surfaces for said film and said grid.

3. An electrode assembly for an electron discharge device comprising afirst member including an annular support member and a thin firstelectrode means upported thereon, a second member including an annularsupport member and a thin second electrode means supported thereon, athin annular spacer member positioned between said first and secondmembers, said spacer members having an inner substantially rectangularaperture, the edges of said aperture lying within the area defined bysaid annular support members, the area of said spacer adjacent the edgesof said aperture bending with successive curves in alternate directionsto provide in elfect a plurality of alternate point support spacersbetween said first electrode means and said second electrode means.

4. A target assembly for an electron discharge device comprising atarget member including a support ring and a membrane supported thereon,a mesh electrode member including a support ring and a mesh supportedthereon, spacer means positioned between said target member and saidmesh electrode, said spacer means providing a substantially rectangularopening with the portion of the surface adjacent the edges of saidopening spacing said mesh and membrane, said surface being wavy in formto provide a plurality of alternate point support spacer surfacesbetween said membrane and said mesh.

5. A target assembly for an electron discharge device comprising a meshelectrode, a target electrode and a spacer member positionedtherebetween, said mesh electrode comprising an annular support ring anda mesh secured to one surface of said support ring, said surface havingan inner annular rib portion and said mesh secured to said surfaceexterior to said rib portion to stretch said mesh over said rib portion,said target electrode comprising an annular support ring and a membranesecured to said support ring of said target electrode, said spacermember including an annular disc-like member having a substantiallyrectangular aperture therein, said spacer bending with successive curvesin alternate directions about said aperture to provide a plurality ofpoint supports for said mesh and membrane, and means clamping said mesh,spacer and membrane together to provide a discontinuous support for saidmembrane to suppress vibration in said membrane.

6. A target assembly for an electron discharge device comprising a meshelectrode, a target electrode and a spacer member positionedtherebetween, said mesh electrode comprising an annular support ring anda mesh secured to one surface of said support ring, said surface havingan inner annular rib portion, said mesh secured I to said surfaceexterior to said rib portion and stretched over said rib portion, saidtarget electrode comprising an annular support ring and a membranesecured to said support ring of said target electrode, said spacermember including an annular outer portion and an inner portion, saidinner portion having a rectangular aperture therein, said inner portionof said spacer member positioned between said mesh and said membrane,said inner portion having an undulating surface for providing a spacersupporting said membrane by a plurality of points. I 7. A targetassembly for an electron discharge device comprising a mesh electrode, atarget electrode and a spacer member positioned therebetween, said meshelectrode comprising an annular support ring and a mesh secured to onesurface of said support ring said surface having an inner annular ribportion, said mesh secured to said surface exteriorly to said ribportion and stretched over said rib portion, said target electrodecomprising an annular support ring and a membrane secured to one surfaceof said support ring of said target electrode, said spacer memberincluding an annular disc-like member having an annular outer portionlying in a first plane and an inner portion having a rectangularaperture therein and lying in a plane parallel to said first plane, saidinner portion separating and spacing said membrane and said mesh toprovide a plurality of support points for said membrane.

8. A target assembly for an electron discharge device comprising a meshelectrode, a target electrode and a spacer member positionedtherebetween, said mesh electrode comprising an annular support ring anda mesh secured to one surface of said support ring, said targetelectrode comprising an annular support ring and a membrane securedthereto, a thin annular disc-like member of about 2 mils in thicknesshaving a rectangular aperture therein and having an undulating portionsurrounding said aperture, said undulating portion separating said meshand membrane by a distance of greater than 2 mils.

JAMES W. LAWRENCE, Primary Examiner.

V. A. LAFRANCHI, Assistant Examiner.

4. A TARGET ASSEMBLY FOR AN ELECTRON DISCHARGE DEVICE COMPRISING ATARGET MEMBER INCLUDING A SUPPORT RING AND A MEMBRANE SUPPORTED THEREON,A MESH ELECTRODE MEMBER INCLUDING A SUPPORT RING AND A MESH SUPPORTEDTHEREON, SPACER MEANS POSITIONED BETWEEN SAID TARGET MEMBER AND SAIDMESH ELECTRODE, SAID SPACER MEANS PROVIDING A SUBSTANTIALLY RECTANGULAROPENING WITH THE PORTION OF THE SURFACE ADJACENT THE EDGES OF SAIDOPENING SPACING SAID MESH AND MEMBRANE, SAID SURFACE BEING WAVY IN FORMTO PROVIDE A PLURALITY OF ALTERNATE POINT SUPPORT SPACER SURFACESBETWEEN SAID MEMBRANE SAND SAID MESH.